What Is a Double Glazed Unit?

 

A double glazed unit normally consists of:

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• Two panes of glass

• A sealed space between the panes

• A spacer bar around the perimeter

• Moisture-absorbing material within the spacer system

• Sealed edges

• Dry air or insulating gas within the cavity

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The complete assembly is sometimes called an insulated glass unit, sealed unit or double glazed panel.

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The panes and spacer bar are permanently sealed together during manufacture. They are then fitted into a window or door frame and held in position with glazing beads, gaskets or suitable glazing compounds.

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How Double Glazed Window Units Work

 

The cavity between the panes helps reduce heat transfer through the window. Modern units may also contain argon gas and low-emissivity glass to improve thermal performance.

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A typical energy-efficient unit may include:

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• Clear external glass

• An argon-filled cavity

• Low-e internal glass

• A warm-edge spacer bar

• Insulated perimeter seals

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Different glass types can also be incorporated depending on the window’s location and purpose.

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These may include:

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• Toughened safety glass

• Laminated security glass

• Obscure privacy glass

• Acoustic glass

• Solar-control glass

• Decorative or patterned glazing

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The correct specification should be confirmed before the replacement unit is manufactured.

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Why Do Double Glazed Units Fail?

 

The most common problem is failure of the perimeter seal.

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Once the seal breaks down, moisture can enter the cavity between the two panes. This may cause the glass to appear:

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• Misted

• Cloudy

• Fogged

• Streaked

• Permanently damp between the panes

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The amount of condensation may change according to the weather and temperature. It can sometimes disappear temporarily before returning.

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Other reasons for replacement include:

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• Cracked or broken glass

• Deep scratches

• Damage around the glass edges

• Failed decorative panels

• Poor thermal performance

• A need for safety or privacy glass

• Accidental impact

• Damage caused during other building work

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A failed sealed unit cannot normally be successfully repaired by cleaning between the panes because the glass assembly is permanently sealed.

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Double Glazing Glass Replacement

 

Double glazing glass replacement usually involves removing the failed unit and fitting a newly manufactured sealed unit into the existing frame.

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The installer will measure:

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• Overall glass width

• Overall glass height

• Total unit thickness

• Individual pane thicknesses

• Cavity width

• Glass type

• Spacer-bar colour

• Glazing pattern

• Safety-glass requirements

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Accurate measurement is essential. A unit that is slightly too large may not fit, while one that is too small may be inadequately supported or sealed.

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The replacement must also be packed correctly within the frame to prevent movement and maintain the alignment of opening windows and doors.

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Can the Glass Be Replaced Without Changing the Frame?

 

Yes. In many cases, replacement double glazed units can be installed while retaining the existing window frame.

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This may be appropriate where:

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• The frame remains structurally sound

• The window opens and closes correctly

• Hinges and locks remain serviceable

• The damage is limited to the glazing

• The sealed unit has become misted

• The glass specification needs upgrading

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Replacing only the unit can be less disruptive and less expensive than replacing the complete window.

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However, the frame should be inspected first. New glass may not represent good value where the frame is warped, cracked, insecure or approaching the end of its useful life.

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Replacing a Misted Double Glazed Window Unit

 

Misting between the panes normally indicates sealed-unit failure rather than a problem with room ventilation.

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The usual process for replacing a double glazed window unit is:

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1. Inspect the frame and confirm that it is suitable for reuse.

2. Measure the existing sealed unit.

3. Identify the required glass specification.

4. Manufacture the new unit.

5. Remove the glazing beads or retaining system.

6. Carefully take out the failed unit.

7. Inspect and clean the glazing rebate.

8. Install suitable setting and glazing blocks.

9. Fit the new sealed unit.

10. Replace the beads and check the window operation.

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The replacement should restore the clear appearance of the glass and improve its thermal performance where the previous seal had failed.

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Condensation Between or On the Glass

 

It is important to identify where the condensation is forming.

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Condensation between the panes

This normally indicates failure of the sealed unit. The unit will usually need replacing.

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Condensation on the internal surface

This is commonly linked to indoor humidity, ventilation, heating and cold surfaces. Replacing the sealed unit may help where the existing glass performs poorly, but it may not resolve the underlying moisture problem.

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Condensation on the external surface

External condensation can occur on high-performance glazing because the outside pane remains colder. It does not necessarily indicate a fault.

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A glazing specialist can establish whether the problem is caused by sealed-unit failure or general household condensation.

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Replacement Glass for Windows and Doors

 

Double glazed panels are used in many different products, including:

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• Casement windows

• Sash windows

• Tilt-and-turn windows

• Bay windows

• Entrance doors

• French doors

• Sliding patio doors

• Bifold doors

• Conservatories

• Rooflights

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Door and low-level glazing may require toughened or laminated safety glass. Large sliding and bifold panels may also need thicker glass and carefully designed support.

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The weight of the replacement unit must be suitable for the frame, hinges, rollers and locking system.

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Safety Glass Requirements

 

Certain locations require safety glazing because there is a greater risk of someone falling against the glass or striking it accidentally.

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Safety glass may be required in:

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• Glazed doors

• Panels beside doors

• Low-level windows

• Sliding and bifold doors

• Bathrooms

• Large glazed panels

• Areas protecting against a fall

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Toughened and laminated glass behave differently when broken. The installer should select the appropriate product according to the location and required performance.

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The original unit may not meet current expectations, particularly where an older window or door is being reglazed.

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Improving the Glass Specification

 

Replacing a failed unit can provide an opportunity to improve the glazing without replacing the frame.

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Possible upgrades include:

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• Low-e energy-efficient glass

• Argon-filled cavities

• Warm-edge spacer bars

• Laminated security glass

• Toughened safety glass

• Acoustic glazing

• Obscure privacy glass

• Solar-control glass

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The available improvements will depend on the glazing thickness the existing frame can accommodate.

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A thicker or heavier unit should not be installed without checking the frame, beads, hinges and opening mechanisms.

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Double Glazed Panels for Doors

 

Large double glazed panels in patio, sliding and bifold doors require careful measurement and installation.

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These units may be considerably heavier than standard window glass. The installer should check:

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• Door frame condition

• Rollers or hinges

• Glazing-platform support

• Safety-glass specification

• Glass-retention system

• Panel alignment

• Locking operation

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Replacing the glass will not correct a dropped or distorted door unless the frame and hardware are also adjusted.

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Where a door has become difficult to operate, both the glazing and mechanism should be inspected.

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Decorative Double Glazed Units

 

Decorative glazing can also be manufactured as sealed double glazed units.

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Options may include:

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• Georgian bars

• Leaded designs

• Coloured glass

• Etched patterns

• Bevelled glass

• Decorative entrance-door panels

• Obscure glass

• Integral blinds

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Matching an older decorative unit can be more difficult than replacing standard clear glass. Photographs, measurements and samples may be needed to reproduce the design as closely as possible.

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Small differences in colour, texture or pattern may remain visible between old and new units.

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How Much Does Double Glazing Glass Replacement Cost?

 

The cost depends on:

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• Unit dimensions

• Glass thickness

• Double or triple glazing

• Safety-glass requirements

• Laminated or acoustic glass

• Obscure or decorative designs

• Solar-control coatings

• Access and working height

• Frame condition

• Number of units being replaced

• Whether hinges or locks also require attention

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A small standard window unit will normally cost less than a large sliding-door panel, shaped unit or specialist heritage glass.

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The quotation should identify the glass specification, installation work, disposal and guarantee.

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Repair the Unit or Replace the Window?

 

Replacing the glass is normally worthwhile where the frame remains in good condition and the fault is limited to the sealed unit.

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Full window replacement may be more appropriate where:

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• The frame is warped or cracked

• Several sealed units and hardware components have failed

• The window no longer locks securely

• Replacement beads or parts are unavailable

• Water enters through the frame

• The existing windows provide poor overall performance

• The frames are badly discoloured or deteriorated

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A reputable specialist should explain both options and help you compare the likely lifespan and cost.

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Double Glazed Unit Guarantees

 

Replacement sealed units should normally be supplied with a written guarantee.

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Check whether it covers:

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• Failure of the perimeter seal

• Condensation between the panes

• Manufacturing defects

• Glass coatings

• Decorative components

• Replacement labour

• Accidental damage exclusions

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The guarantee may be shorter for decorative units, integral blinds or specialist glass than for standard sealed glazing.

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Keep the invoice and glass specification in case another matching unit is required later.

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Find Trusted Double Glazing Specialists

 

Replacing a failed double glazed unit can restore the appearance and performance of an existing window without the disruption of changing the complete frame.

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APN Double Glazing can connect homeowners with experienced local specialists for misted glass, broken windows, replacement double glazed units and specialist glazing panels.

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Get in touch for expert advice and a free, no-obligation quote for replacing your double glazed window or door units.